Improved hole machining accuracy is required in a drill which is used for drilling in die machining or part machining. Various studies have been made in order to obtain improved machined face quality and machining accuracy.
In the related art, a drill is known in which a cutting edge is formed to be convex in a direction toward a tip of the drill so as to disperse a cutting resistance to obtain a high-quality machined face. PTL 1 discloses a drill in which a cutting edge has a continuous convex shape from a cutting corner portion toward a tip in an axial direction and which has a two-staged flank face.
In addition, various thinning is performed on the tip of the drill in order to decrease a biting load or a thrust load with respect to a work material. PTL 2 discloses a drill which can realize high-accuracy drilling by forming an incision (thinning) to a position beyond a center of a tool so as to form a thinning cutting edge.
PTL 3 discloses a drill which has a main margin and a sub margin and in which a thinning face is formed over the entire region from an inner end of a cutting edge in a radial direction to an outer end thereof in the radial direction to increase a fracture resistance of the cutting edge.